Electrical equipment from various fields of application, e.g. mobile telephones, personal digital assistants (PDA), and industrial control equipment often use a display device of some sort for providing the operator of the device with information. In simpler applications the display device is a one-way communication link, i.e. the display is used for providing information to the operator but not to receive information the other way back. In order to achieve interaction with the operator, push buttons or keyboards are normally used. If the electrical equipment is small sized, for example as with a PDA, normally no room is left on the device for a keyboard, wherein the manufacturer of the PDA must provide other means for enabling input of data into the device.
As is well known in the art the input means may be in form of a touch sensitive display making it possible to enter data without the need for a separate keyboard. Many different techniques for providing touch sensitive devices have been presented and the most common solution today is to use a separate transparent touch sensitive layer which is placed on top of the display. The touch sensitive layer is normally in form of two flexible superimposed plastic sheets that are separated by a small distance by means of insulating spacers. On the surfaces of the sheets facing towards each other, a matrix-like pattern of electrical conductors are arranged which pattern establishes an electric contact between the sheets at the location where the touch sensitive layer is depressed. A control unit scanning the matrix-like pattern on the plastic sheets may then detect the electric contact between the sheets and determine the coordinates for the depression on the display.
Even though the separate touch sensitive layer makes it possible to enter data into the device without the need for a keyboard, it is not an efficient way of realising a touch sensitive display since the transparency of the touch sensitive layer is not absolute hence making it difficult to view the information presented on the display under certain circumstances. The unsatisfactory transparency of the touch sensitive layer is even more noticeable when the display device is provided with back lighter or front lighter technology for making it possible to view the information on the display under poor lit conditions.
Another approach for providing a touch sensitive display is to provide a display with a sensor arranged under the display rather than on top of the display. The sensor then has to detect a touch on the display not by means detecting an electric contact between conductors as with the solution disclosed above, but by using capacitive or reflective properties of the display. In the former case, a capacitive coupling through the display to the finger touching the display makes it possible to detect a touch on the display as well as determine the position of the touch. In the latter case light or sound utilizing changes in the reflective properties of the display at the point of contact may be used for detecting a touch on the surface of the display.
Attempts have been made to provide touch sensitivity for displays without the use of separate sensors arranged on top or below the display surface. An approach is to use the display electrodes forming the pixels or the segments of the characters on the display for sensing the touch.
U.S. Pat. No. 5,043,710 discloses a touch sensor comprising a liquid crystal display (LCD), wherein a touch on the display is sensed by detecting changes in the dielectric properties of the display. A mechanical force applied to the LCD perpendicular to a flexible glass substrate over one of the display electrodes gives rise to a temporary disorganisation of the molecules in the liquid crystal thereby changing the dielectric constant of the liquid crystal under the display electrode. Each display electrode of the LCD is connected to an integrator, wherein a change of the dielectric constant of the liquid crystal when the segments of the LCD are in an excited state gives rise to an electric pulse indicating a touch on the LCD. However, the solution according to U.S. Pat. No. 5,043,710 becomes complex due to the large amount of integrators needed for sensing a touch. Moreover, for sensing a touch the front glass plate needs to be flexible making the display less durable. In addition to this, the working life of the display is also decreased due to the repeated compressions of the liquid crystal in the display.
U.S. Pat. No. 4,224,615 discloses a LCD with a flexible front plate, which LCD may be used as a device for receiving data from a human operator. An operator of a device comprising the touch sensitive display touches the flexible front plate of the display, wherein the front plate deflects towards the back substrate thereby increasing the capacitance between the display electrodes residing in the area being depressed. The capacitance measured between the front and back display segment is compared with the capacitance of a reference cell, wherein it is possible to detect a touch even if the affected display segments are actuated, i.e. presenting a shape on the display. As with U.S. Pat. No. 5,043,710 the invention according to U.S. Pat. No. 4,224,615 uses the change in dielectric constant of the liquid crystal being compressed for sensing a touch. The same problems with robustness and life expectancy as with the invention according to U.S. Pat. No. 5,043,710 exist in the solution according to U.S. Pat. No. 4,224,615.
US 2001/0020578 discloses a LCD with touch sensitivity, wherein the sensor arrangement is placed below a surface of the display. The sensors are preferably placed below the display in the regions of the display where no display segments are arranged. Alternatively, the display segments of the display may be used as sensors provided that the front and back segment are short-circuited. When the display electrodes act as touch sensors, no information may be presented on the screen due to the short-circuiting of the display electrodes. A microprocessor is therefore coupled to the display segments for alternating between presentation of information on the display and touch sensitivity.
U.S. Pat. No. 4,910,504 discloses a touch controlled display device, wherein a touch on the display is sensed by measuring the capacitance between different display electrodes on the front substrate. The font substrate may then be rigid protecting the display from deformation. The detector measuring the capacitance between the electrodes is coupled to the feeding pins of the display. A common counter-electrode is arranged on the back substrate in a manner known per se. As will be disclosed below, the counter-electrode will act as a short-circuit between the electrodes on the front substrate thereby deteriorating the accuracy of the touch sensitive display in regard of where on the screen the touch is made. Moreover, numerous stray-capacitances in the needed drive circuitry for the display will interfere with the capacitance measuring circuitry making it hard to determine where and if a touch is made.
DE 19802479 discloses a touch-sensitive display for use in e.g. elevators. The front surface of display element is provided with an electrically conducting layer which is so thin that the display element is visible through the conducting layer. An evaluation circuit is connected to the conducting layer in order to detect a touch on the display. However, by arranging a conductive layer in front of the display element, the visibility of the display element is deteriorated. Moreover, the conductive layer will be exposed to wear from users of the display, which implies that the endurance of the display will be insufficient for many applications.
For manufacturers of display driver circuits it is of most importance that the circuitry used for detecting a touch on the screen is not affecting the behaviour or the life-expectancy of the driver circuitry. Hence a touch sensitive display which behaves like a “normal” display from a drivers point of view is hence wished for.